The invention relates to a magnetic position detector, and more particularly to a magnetic angular or linear position detector which utilises a moveable target track comprising magnetic data of a first kind and a second kind in conjuction with a magnetically sensitive component arranged with a biasing magnet.
Magnetic position detectors are widely used in dirty environments like automobiles since they are insensitive to dust and other contaminations. A position detector typically consists of a soft magnetic target, a magnetic sensor, and a permanent magnet. The target has a tooth/slot pattern which modifies the magnetic field pattern of the permanent magnet while it is moved along the magnet. The sensor is usually in between the magnet and the target and can detect whether the magnet is facing a slot or a tooth by the difference in magnetic field strength. The sensor is not in contact with the target, which prevents the detector from wear.
Unfortunately, the magnetic field strength detected by the sensor is not only a function of the tooth/slot pattern, but also of the gap between the sensor/magnet pair and the soft-magnetic target. This causes a shift in the detected position of a tooth edge if the gap changes. A conventional Hall crank shaft sensor, in front of a rotating soft magnetic target wheel, for gaps ranging from 0.1 up to 2.4 mm e.g. shows a strong change in output with the length of the gap. In practice, this change in output induces a shift in the detected position of a tooth edge of about 1-2 degrees.
An elegant way to circumvent this gap dependence is the use of two complementary targets (U.S. Pat. No. 5,444,370) in combination with one or two sensors (one for each target), where the sensor can measure the field strength. A non-magnetic spacing between the two complementary targets makes the set-up less sensitive to tolerances in the axial direction (U.S. Pat. No. 5,668,471).
Magnetoresistive sensors, like AMR sensors, GMR (giant magnetoresistive effect) sensors, and spin-tunnel junction sensors have the advantage over Hall sensors that they can be used to measure the direction of the magnetic field generated by the permanent magnet, whereas the Hall sensors can only measure the field strength. The strength of a magnet is dependent on its temperature, which can easily vary over more than 150 degrees in automotive applications. Moreover, AMR and GMR sensors have an in-plane sensitivity, whereas Hall sensors measure field components perpendicular to the thin-film structure.
It is an object of the invention to provide magnetoresistive sensor configurations which benefit maximally from the advantages of these sensors, so as to produce an intrinsically higher spatial resolution. The invention relates to a magnetic position detector. The claimed detector arrangement is such that the output signal of the sensor is mainly determined by the direction of the sensed field and practically not by the strength of the sensed field. A first magnetic datum causes the magnetic field flux to flow in a first, e.g. straight, direction through the sensor, and a second magnetic datum causes the flux to flow in a second direction, which is e.g. bended relative to the first direction.
Several embodiments are presented, some of which also show less influence of axial gap variations. The proposed configurations have in common:
1. The direction of the magnetic field at the location of the sensor, as generated by the bias magnet, is predominantly in the plane of the sensor surface both when the sensor is next to a magnetic datum of a first kind, e.g. a non ferromagnetic segment (slot) and next to a magnetic datum of a second kind, e.g. a ferromagnetic segment (tooth) of the target track.
2. The sensor detects the change in the direction of the magnetic field emanating from the magnet caused by the tooth-slot pattern of the soft magnetic track on the target wheel.
3. Preferably, the surface of the (thin film) sensor is perpendicular to the direction of movement of the magnetic target, in order to get the maximum spatial resolution.
4. Preferably, the sensor is in or near the saturation mode, both when the sensor is facing a slot and when it is facing a tooth.